An investigation of the cell surface properties of cells transformed by simian virus 40 and polyoma virus showing loss of their transformed properties. Cells showing total or partial reversion of transformed properties have been selected on the basis of a reduced susceptibility to agglutination to plant lectins, low reactivity to Forssman antisera and by the BUDR method for isolating cells having a slower growth rate than the majority of the transformed cell population. The selection of other revertant cell strains and lines is also in progress using cell hybrids formed by fusion of virus-transformed cells with various normal cell types.